Not applicable.
The demand for high precision positioning systems, or stages has grown rapidly in some key industries. Applications for such systems include semiconductor manufacturing equipment, high precision machining optics (lenses and mirrors) and mass data storage. For example, semiconductor manufacturing equipment will soon require 0.1-1 nm precision, and 30xc3x9730 cm2 travel range.
A friction drive system has a simple structure as described in a paper entitled Transmission Systems of Motion and Force-Friction Drive/Traction Drive, K Adachi et al., Journal of Japan Society of Precision Engineers, Vol. 60, No. 10, pp. 1410-1415, 1994, which is incorporated herein by reference. Since no gear reduction unit is used, no backlash exists. The position of the stage, which often takes the form of a platform or table, is very stable due to a large static frictional force. Friction drive systems do not require a lubricant and therefore, can be adequate for use in clean environments. However, a friction drive requires strong wear resistant materials of which many reliable types have been reported.
Friction drive is appropriate for applications requiring small load but high precision. In practice, many one dimensional or two dimensional (hereinafter 1-D or 2-D, respectively) high precision friction drive stages have been developed because of the advantages listed above. 1-D high precision friction drive positioning stages with nanometric precision are described in: Nanometric Motion Control of a Traction Drive, P. I. Ro et al., Dynamic Systems and Control, Vol. 2 ASME, pp. 879-883, 1994 and Precision Table Control System by Friction Drive for Optical Disk Mastering Machine, S. Sakuta et al., Journal of Japan Society of Precision Engineers, Vol. 62, No. 10, pp. 1444-1448, 1996. Most of the developed high precision positioning stages which use friction drive are usually actuated in 2-D by the use of two 1-D stages as described in The Micropositioning Book, Burleigh Instruments, Inc., Burleigh Park, Fishers, N.Y., 14453, 1990 and Ultrasonic Linear Motor for a High Precision X-Y Stage, K. Mori et al., Proceedings of Ultrasonic Symposium, pp. 657-660, 1989 and Nikkei Mechanical, No. 507, pp. 74-79, May 26, 1997 (in Japanese). For three degree of freedom motion such as X-Y-xcex8 motion, the stage system usually uses three 1-D stages. Consequently, the system""s structure and control scheme are more complex. In addition, the weight of the stage system itself is larger and more expensive.
On the other hand, some innovative 2-D friction driven stages, using only one stage, have been proposed by several researchers. However, most of the designs do not allow diagonal motion as is the case in systems described in: New Multi-Degree of Freedom Piezoelectric Micromotors for Micromanipulator Applications, A. Ferreira et al., Proceedings of IEEE Ultrasonic Symposium, pp. 417-422, 1995 and A Linear Ultrasonic Motor for Nano-Technology, H. Choi, Master""s Thesis, Massachusetts Institute of Technology, 1996. Each move is either in the X or Y direction at any one time, resulting in a slow rectangular motion. The reason for this is that the motion in one direction prevents the motion in the other direction.
A 2-D stage design, which uses one stage and is capable of diagonal motion, was achieved by using a 2-D surface acoustic wave as described in Ultrasonic Linear Motor using Surface Acoustic Waves, M. Kurosawa et al., TEEE Trans. on Ultrasonics, Ferroelectrics, and Frequency Control (UFFC), Vol. 43, No. 5, pp. 901-906, 1996. However, this paper reported that the stage is not reliable because it has an aging problem due to very small vibration amplitude (5 nm), and the motion is unstable. Further, this system may require expensive power electronic components due to its high operating frequency (10 MHZ). N. Hoshi et al. proposed an array type of 2-D stage which can move diagonally in a paper entitled Analysis of Plane Ultrasonic Piezoelectic Actors, IEEE Industry Applications Magazine, July/August, pp. 24-29, 1995. In principle, this system can only move semi-diagonally because the number of movement direction choices is limited. Further, its cost may be high due to a complex structure.
According to the invention, an omni-directional high precision friction drive positioning stage system is provided. The proposed system has only one movable stage capable of omni-directional movement with three degrees of freedom. The three degrees of freedom include two linear (X and Y) and one rotational (xcex8) about the Z axis. Stated differently, the stage can move in X and Y (i.e., any direction in the plane of stage) and can also be rotated in the plane of the stage about the Z axis. In one embodiment, the stage uses three special actuation systems, each of which is designed to generate a directional elliptical motion which allows the stage to move in X, Y and xcex8. The design has a simple structure, low cost, and high precision.
The friction drive positioning stage system includes a substantially planar stage and a plurality of actuation systems, each having a tip in contact with a surface of the stage. The tip of each such actuation system is movable in an elliptical path disposed in a rotatable plane substantially orthogonal to the plane of the stage. Stated differently, the tip of each such actuation system is capable of directional elliptical motion. Each of the actuation systems is independently controllable in order to permit the stage to be moved in 2-D (X and Y) as well as rotated (xcex8). In one embodiment, the friction drive positioning stage system includes three such actuation systems, each one having a tip in contact with the stage.
Also described is an actuation system suitable for use with the above-described friction drive system. The actuation system includes a plurality of elongated actuation members and a substantially rigid hat-shaped element attached to an end of each of the elongated members and having a rounded portion at which the tip is disposed. Each of the elongated actuation members is preferably angularly spaced from the other such members by 120 degrees. The elongated actuation members are comprised of a piezoelectric material.
In accordance with a further aspect of the invention, a method for moving a stage having a substantially planar surface in 2-D and in rotation includes the steps of providing a plurality of actuation systems, each having a tip which is movable in an elliptical path disposed in a rotatable plane substantially orthogonal to the plane of the stage. The method further includes the step of independently controlling the angle of the plane of the elliptical path of each actuation system.
With the above-described apparatus and techniques, an improved friction drive system is provided which is capable of moving the stage in 2-D as well as in rotation in a simple and cost effective manner. The omni-directional (X, Y, xcex8) positioning system of the present invention has advantages of simple structure, low cost, as well as high precision positioning capability, such as precision on the order of one nanometer or less.